<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let signInfo = [
      {
        fieldId: 539,
        value: undefined
      },
      {
        fieldId: 540,
        value: undefined
      },
      {
        fieldId: 546,
        value: undefined
      },
    ]

    // 判断到value为undefined，返回空字符串即可
    const res = JSON.stringify(signInfo, (key, value) => {
      console.log(key, value)
      return  value
    })
    console.log(res)
    // '[{"fieldId":539,"value":""},{"fieldId":540,"value":""},{"fieldId":546,"value":""}]'
  </script>
</body>
</html>